KR100188049B1 - Process for poducing high-strength stainless steel strip - Google Patents

Abstract

A high strength steel strip excellent in shape having a duplex structure of austenite and martensite has been prepared by a process which comprises providing a cold rolled or cold rolled and annealed strip of a martensitic structure from low carbon martensitic stainless steel containing from 10 to 17 % by weight of Cr and having a carbon content of not exceeding 0.15 % by weight, causing the strip to continuously pass through a continuous heat treatment furnace where the strip is heated to temperatures within the range from (the As point of the steel + 30 DEG C.) to the Af point of the steel and not higher than 900 DEG C. so that a part of the martensitic phase may be changed to a reversed austenitic phase, and cooling the heated strip to ambient temperature, wherein the As point of the steel is a temperature of the steel of which temperature is being raised at which the transformation of martensite to austenite begins and the Af point of the steel is a temperature of the steel of which temperature is being raised at which the transformation of martensite to austenite is finished.

Description

Manufacturing method of high strength stainless steel strip with excellent shape

1 is a perspective view showing a measurement method in the L direction (rolling direction) of the shape measurement method of the strip before and after heat treatment.

2 is a perspective view showing a measuring method in the T direction (perpendicular to the rolling direction) of the strip shape measuring method before and after heat treatment.

The present invention relates to a method for producing a high strength stainless steel strip having excellent shape. As high-strength stainless steels having a tensile strength of 100 kg / mm2 or more, pore-cured austenitic stainless steels, low carbon martensitic stainless steels, and precipitation-curable stainless steels are known. Because of its excellent heat resistance and the like, it is widely used in the manufacture of steel belts and various springs by utilizing its characteristics. For example, a material for a steel belt and a method of manufacturing a steel belt are disclosed in Japanese Patent Publication No. 51-1085 and Japanese Patent Publication No. 61-9903.

Processed softened austenitic stainless steel strips are manufactured by cold rolling semi-stable austenitic stainless steels to impart a working strain and high strength (tempering) cold rolled strips. Site-based stainless steel strips are produced by quenching low-carbon Cr-Ni-based stainless steels whose composition is adjusted to exhibit martensite structure at room temperature from a typical annealing temperature of 900 ° C. or higher. . All of these strips require a rolling process for shape correction using a rolling mill equipped with a roll having a large diameter in the final process in order to obtain an excellent shape. Such shape correction rolling is performed in both the work hardening austenitic stainless steel and the martensitic stainless steel, in order to obtain a good shape in the rolling ratio, the roller diameter, the rolling speed, the sheet thickness, the type of steel, and the previous process. It is necessary to perform properly in consideration of the history, etc., the success or failure of the product will greatly affect the product yield or it will be impossible to obtain a flat stainless steel strip. Therefore, it is preferable that there is no final rolling process for such shape correction as much as possible, but in the above-described strip, this cannot be avoided in the manufacturing history.

The present invention has been made in view of the above-mentioned problem to omit the final rolling process for shape modification and to provide a method for producing a high tensile strength stainless steel strip of 100kg / ㎜ or more having an excellent shape.

The present invention provides a cold rolled steel strip or cold rolled annealing steel having a martensitic phase from a low carbon martensitic stainless steel containing 10 to 17 wt.% Of Cr and having a C content of 0.15 wt.% Or less. A strip is produced, and the strip is continuously passed through a continuous heat treatment furnace to be heated to a temperature within the temperature range (above As point + 30 ° C.) to below the Af point of the steel (but not more than 900 ° C.) to martensite. A part of the phase is transformed into an austenite phase and then cooled at room temperature to produce a strip having an austenite phase and a martensite phase morphology. It is possible to produce high strength stainless steel strips of excellent shape that do not require shape modification rolling in subsequent processes. And the As point of the steel is the temperature at which the transformation starts from the martensite phase to the austenite phase during the temperature raising process, and the Af point of the steel is the temperature at which the transformation finishes from the martensite phase to the austenite phase during the temperature raising process. Indicates.

In the practice of the present invention, a better result is obtained by easing the tension applied to the steel strip passing through the continuous heat treatment furnace from the low temperature side to the high temperature side. This tension control can be advantageously carried out by adjusting the tension by the weight of the strip itself, ie the distance between the support rollers supporting the strips in the furnace, while passing through the continuous heat treatment furnace. The strip before passing through the continuous heat treatment furnace may have a ferrite phase or an austenite phase of 20 vol.% Or less in addition to the martensite single phase structure.

In the manufacturing method of the present invention, the stainless steel strip passing through the continuous heat treatment furnace is continuously heated while applying tension in the longitudinal direction (LD) of the strip. The continuous heat treatment method of the present invention, which heat-treats the strip while applying the tension, has a large difference from the batchwise heat-treatment method which heats the coil-shaped strip without applying tension. When a stainless steel strip with martensite structure is heated to a temperature above the As point of the steel in a continuous heat treatment furnace, the martensite is reverse transformed into austenite with tension applied in the longitudinal direction of the strip. That is, since the reverse transformation progresses while the tension acts in the direction in which the material is flattened, the material is flattened according to the reverse transformation progression. If the heat treatment temperature does not exceed 900 ℃ and is within the temperature range from (As point + 30 ℃) to Af point or lower, it is possible to reverse part of martensite phase into austenite phase. Since the austenite phase is a fine and extremely stable phase, it does not transform into quenched martensite even when cooled to room temperature.

Therefore, the steel strip produced by the method of the present invention can be made into a fine two-phase structure between the martensite phase and the reverse transformation austenite phase and has high strength.

In addition, the fact that the reverse transformation austenite does not transform again into quenched martensite between cooling from the continuous heat treatment temperature means that no strain due to quenching occurs, which results in good flatness of the strip achieved in the heat treatment furnace. Can be maintained at room temperature.

A continuous heat treatment furnace for carrying out the method of the present invention may use a catenary furnace, a vertical furnace, or the like, which is commonly used for annealing a strip. The atmosphere of the furnace is preferably a reducing gas or an inert gas in consideration of anti-oxidation of the strip. At this time, the furnace may be heated in a manner such as a heating element heating method, a combustion method such as gas or oil, or the like. In the continuous heat treatment according to the present invention, a tension is inevitably applied in the longitudinal direction of the strip, and the degree of tension is preferably higher than 0.5 kgf / mm 2 at the low temperature near the As point of the steel, and high temperature near the Af point. The lower tension of 0.5 kgf / mm <2> or less is preferable. The adjustment of this tension can be conveniently carried out by adjusting the distance between the supporting rolls supporting the strips in the furnace as described above.

According to the characteristics of the present invention, during the continuous heat treatment in this way, a fine austenite phase is reversely transformed from the martensite phase into a fine two-phase structure, and a high-strength stainless strip having excellent shape is produced by maintaining the fine two-phase structure. You can do it. Therefore, it is essential to obtain a stable and fine two-phase tissue. (As point of steel + 30 ° C). In the heat treatment in the following temperature range, the amount of reverse-pathogenic austenite produced is insufficient, and in the heat treatment in the temperature range exceeding the Af point or in the temperature range of 900 ° C or more, the amount of reverse transformation austenite increases so that the amount of martensite does not remain. Or very little residual amount, thus making it impossible to obtain a stable and fine two-phase structure. Therefore, such continuous heat treatment should be carried out in the temperature range (not more than 900 ℃) below the steel Af point above (as point of steel + 30 ℃).

Stainless steel used in the present invention is a martensitic stainless steel exhibiting a martensite structure in the annealed state. The structure of the steel strip prior to passing through the continuous heat treatment furnace must be substantially martensite, which is also related to the composition of the steel, and in the final annealing step annealing steel strips of martensite structure, The cold rolled steel strip in which the annealing steel strip is finished cold rolled, in some cases, cold rolled steel strip in which strain organic martensite is produced by cold rolling may be used as the starting strip. However, the structure of the steel strip before the heat treatment does not necessarily have to be 100% martensite phase, and a ferrite phase or an austenite phase of up to 20% by volume may be present. In any case, one of the objects of the present invention is a strip having a high strength of 100 kgf / mm2 or more in the state of a delamination structure produced by continuous heat treatment, and the composition and structure ratio of steel in a range that satisfies these requirements. It is included in the present invention.

Regarding the components of the steel, low carbon martensitic stainless steel containing 10 to 17 wt.% Chromium (Cr) and 0.15 wt.% Or less carbon (C) is mainly used in the present invention. Nickel (Ni) can also be one of the main components, and of course, addition of other alloying elements typically contained in low carbon martensitic stainless steel to this kind of steel is possible as long as the above requirements are met.

The typical chemical component and content are as follows.

C: 0.15% or less (not including 0),

Si: 6.0% or less (not including 0),

Mn: 10.0% or less (not including 0),

Ni: 8.0% or less (not including 0),

Cr: 10.0-17.0%

N: 0.3% or less (not including 0),

Mo: 4.0% or less (including 0),

Cu: 4.0% or less (including 0),

Co: 4.0% or less (including 0).

In addition, the steel used in the present invention may contain Ti, Al, Nb, V, Zr, B and 1.0% or less of the total amount of rare earth elements and inevitable impurities.

In addition, the amount of each alloying element is adjusted to each other so that the nickel equivalent (Ni _eq ) in the steel is in the range of 8.0 to 17.5, and the nickel equivalent in the steel is Ti, Al, Nb, V, Zr, B and rare earth elements in the steel. If it does not contain at all is defined as follows,

When steel contains any of Ti, Al, Nb, V, Zr, B and rare earth elements, it is defined as follows.

The numerical ranges of the content ranges of these main elements are thus limited for the following reasons.

C is an austenite-forming element, which not only acts effectively to stabilize the reverse austenite phase produced during heat treatment in a temperature range above (As point of steel + 30 ° C) to below the Af point of steel, but also reverse austenite phase It is effective for strengthening phase and martensite phase. However, when contained in a large amount, Cr carbides are formed during heat treatment, and the corrosion resistance of the steel is poor, so the upper limit thereof is 0.15%.

Cr is a basic component of stainless steel, and in order to obtain good corrosion resistance, it is necessary to contain Cr at least 10.0%. However, if Cr is a ferrite generating element and is contained in a large amount, a large amount of δ ferrite phase is produced, so that it is difficult to obtain martensite single phase structure at room temperature after annealing, so the upper limit is 17.0%.

Ni is an austenite forming element and acts effectively for stabilizing the reverse transformation austenite phase produced during heat treatment in a temperature range above (As point of steel + 30 ° C) to below Af point of steel. However, if it is contained in a large amount, since martensite single phase structure at room temperature becomes difficult to obtain after annealing, it is preferable to contain it in the range of 8.0% or less.

Si widens the temperature range of the As point and the Af point, which is advantageous in obtaining a stable two-phase structure in the austenite phase and the martensite phase, and also in the reinforcement of the reverse transformation austenite phase and martensite phase generated during heat treatment. It is a valid element. However, if it is contained in a large amount, the manufacturability of the steel strip is weakened.

Mn is an austenite forming element and works effectively for stabilizing the reverse transformation austenite phase generated during heat treatment in a temperature range of (above As point of steel + 30 ° C) to below Af point of steel. However, when it is contained in a large amount, since manufacturability deteriorates, for example, Mn fume is formed at the time of melting, it is preferable to set it as 10.0% or less.

N, like C, is an austenite forming element that not only acts in stabilizing the reverse transformation austenite phase formed during heat treatment in the temperature range above (As point + 30 ° C) to below the Af point of steel, but also reverse transformation oh It is an element that also effectively acts to strengthen the steritic phase and martensite phase. However, if it is contained in a large amount, it is preferable to set it to 0.30% or less since a blow hole is formed in the solvent and a healthy ingot cannot be obtained.

Mo not only improves the corrosion resistance of the steel, but is also an effective element for reinforcing the reverse transformation austenite phase and martensite phase generated during heat treatment. However, if Mo is a ferrite generating element and contains a large amount, it is preferable to make a large amount of δ ferrite, so that it is difficult to obtain martensite single phase structure at room temperature after annealing.

Cu, like Ni, is an austenite forming element and is effective for forming an austenite phase during heat treatment. However, when Cu is contained in a large amount, it is preferable to set it to 4.0% or less.

Co, like Ni, is an austenite forming element and is effective for forming an austenite phase during heat treatment. However, Co is preferred to be 4.0% or less because the steel becomes expensive.

Ti, Al, Nb, V and Zr are all effective in maintaining the two-phase structure of austenite and martensite produced by reverse transformation treatment as stable, fine and uniform structure, and also inhibit the formation of Cr carbide. It is also an effective element to maintain corrosion resistance. However, when a large amount is contained, the production of steel strips becomes difficult, so it is preferable that the amounts of these elements be 1.0% or less, respectively, and the total amount thereof be 1% or less.

Ni _eq (Ni equivalent value) is as follows.

As described above, in the method of the present invention, when the heat treatment is reversed from the martensite phase, a fine austenite phase is formed to form a fine two-phase structure, and by maintaining the fine two-phase structure, high strength stainless steel having excellent fatigue properties is produced. do. Therefore, in the present invention, it is essential to obtain a stable and fine two-phase tissue. Steel nickel equivalent (Ni _eq), even if a heat treatment at a relatively low temperature in a 8.0 less than if the temperature range from more than (As point + 30 ℃) to below the river Af point inverse transformation O-stearyl nitro amount becomes insufficient, and the Ni _eq When it exceeds 17.5, since the amount of reverse transformation austenite becomes excessive, it becomes difficult to obtain a stable and fine two-phase structure in any case. Therefore, it is preferable to adjust each amount of components so that the Ni _eq of steel may be 8.0-17.5.

EXAMPLE

The steel with the composition shown in Table 1 is 6 mm thick by solvent, forging and hot rolling according to the usual method, and subjected to cold rolling and annealing after solution treatment and pickling. Next, cold-rolled cold-rolled material having a thickness of 1 mm was made by finishing cold rolling at a predetermined rolling rate. At the time of cold rolling, in order to confirm the beneficial effect of the shape correction during the heat treatment according to the present invention, a condition in which the rolled shape was badly set was set. A part of the finished cold rolled material was annealed at 1030 ° C., followed by pickling to prepare a test material. Table 1 also shows the As and AF transformation points of the steels used for the test. These transformation points are electrical resistance measuring devices, and the temperature-electric resistance relation curves obtained by heating each specimen at a heating rate of 1 ° C./min are used. It calculated | required from the inflection point.

These specimens were treated in a continuous heat treatment furnace under the various conditions listed in Table 2. The passage speed of the steel strip during continuous heat treatment was adjusted so that the residence time in the furnace was 6 minutes. After continuous heat treatment, the shape test, strength and tensile test were performed. The shape inspection was carried out before and after the heat treatment. As shown in FIG. 1, the value obtained by dividing the elevation height h (mm) by the distance l in mm in the rolling direction was defined as the shape value in the L direction. As shown in the figure, the value obtained by dividing the elevation height h (mm) by the width l (300 mm) of the steel strip was defined as the shape value in the T direction. These measurement results are shown in Table 2. As can be seen from the results of the second table, according to the method of the present invention, each test agent has a high strength of 90 kgf / mm 2 or more, and the shape value is 2/1000 or less in the L direction and 1.5 / 300 or less in the T direction. As it has an excellent shape.

On the contrary, the test materials produced by the comparative method of the heat treatment conditions out of the range specified in the present invention in the second table (test Nos. 2, 6, 9, 14 and 15) are poor in shape and have low strength.

As described above, according to the present invention, a highly sensitive stainless steel strip having excellent shape can be produced without performing shape correction rolling. In the production of high strength stainless steel strip of 100 kgf / mm 2 class, the fact that the shape modification rolling can be omitted can greatly contribute to the shortening of the process step and the improvement of the production yield.

In addition, the steel strip produced in the present invention can provide a material useful as a belt material or a spring material in that the steel strip has not only high strength but also a double layer structure, and thus has excellent fatigue strength.

Claims (1)

From a low carbon martensitic stainless steel containing 10 to 17 wt.% Cr and a C content of 0.15 wt.% Or less, a cold rolled steel strip or annealing steel strip having a martensite phase is prepared, and the steel strip is subjected to continuous heat treatment. Passed continuously through the furnace and heated to a temperature within the temperature range (from 900 ° C or lower) from above (As point of steel + 30 ° C) to below the Af point of steel, but a part of martensite phase was transformed into reverse austenite phase. And then cooled to room temperature to form a steel strip having an austenite phase and martensite phase structure, and the steel strip passing through the continuous heat treatment furnace was heated from the low temperature side to the high temperature side. ) The tension is reduced, and the suspension tension is adjusted according to the distance between the rollers supporting the steel strip. Process for producing high strength stainless steel strip with superior phase. However, the As point is the temperature at which the transformation starts from the martensite phase to the austenite phase during the temperature raising process, and the Af point is the temperature at which the transformation is completed from the martensite phase to the austenite phase during the temperature raising process.